A common form of thermal sensor for laser power measurement is based on a thermopile, disposed on the outer periphery of a thermally conductive disc, so that it measures the radial temperature gradient of the disc at the outer periphery. FIG. 1 illustrates such a prior art disc, with the laser beam 15 impinging in the central section of the disc and the disc cooled around its periphery 12. The heat flows radially, causing a temperature drop between the hot and cold thermocouple junctions thus producing a voltage proportional to power input. The thermopile 10 is located towards the outer edge of the disc, and the cooling region is at the periphery 12, outside of the thermopile. FIG. 2 shows such a disc 20 mounted in a laser power meter head, with its outer edge 22 in good thermal contact with the body of the head 23, which can be air or water cooled, and the heat flow across the thermocouple 10.
The advantage of this arrangement is that the power reading is largely independent of beam position and size as long as the beam is inside the inner junctions of the thermopile ring. The disadvantage is that even with time response accelerating circuits and software, the response time of such a sensor is typically no faster than 1 sec. for reaching 95% of the final reading. The reason for this is the need for the heat deposited onto the center of the disc to travel by conduction radially to the cooled outer region. Typically, for a disc designed to measure powers of the order of a few tens of Watts, this distance is of the order of 10 mm In addition, due to the relatively long heat path, the power handling capacity of such a sensor may be limited.
There therefore exists the need for a thermopile power sensor that is able to handle at least the high power levels of prior art radial flow thermopile power sensors, but which has a substantially faster response time than that of such prior art radial flow sensors.
The disclosures of any publications mentioned in this section and in other sections of the specification, are hereby incorporated by reference, each in its entirety.